Put your phone on the DNC List (Do Not Call List), it's a National List, once it's put there advertisers are not allow to contact you with sales call and I believe that includes text messages since that does not use the Data Network but the Phone Voice Network.

If they violate that, I believe you can sue for $5000 that's the violation fee!

And in the Sprint page you can block numbers or allow numbers.

So if you know all the people you want to allow, put those in and all others are blocked.